Though hundreds of metallic alloys have been developed and employed in the handling of sulfuric acid, relatively few of them are useful in handling hot concentrated sulfuric acid. There are instances in which high hardness or resistance to erosion or abrasion coupled with corrosion resistance is very desirable, such as when vapor bubbles and/or grit are present. The commercial hard alloys for concentrated sulfuric acid handling have been the two nickel base alloys disclosed by U.S. 2,938,786 and U.S. 2,938,787 as well as the high-nickel, cobalt-bearing, low-iron alloy disclosed by U.S. 3,758,296. These alloys are all quite expensive and very brittle.
In British Pat. No. 1,534,926 a broad range of alloy compositions are suggested but examples are only provided of low-nickel stainless steel alloys, said to have excellent resistance to concentrated sulfuric acid. In particular, Table 3 of that patent discloses alloys of 10-17.6% Cr, 15-20% Ni, 4.6-9% Si, 1-2% Cu and 0.8-1% Mo.
According to the '926 patent, the disclosed alloys, besides an austenitic character, may, under certain circumstances be partly ferritic. The '926 patent teaches that it is preferable to have a silicon content of 6.5-12to give completely satisfactory corrosion resistance and that generally the lower limit is 7% because at higher silicon contents (&gt;11%) silicon-containing intermetallic phases, understood to be primarily silicides, are formed very rapidly at certain temperatures thereby reducing the workability of the steel, that is, the steel becomes very brittle.
While it is stated in the '926 patent that the steel can normally contain a maximum of 0.06% C., it is also taught that the carbon content should usually be lower, as for example a maximum of 0.02%. It is further taught that above 0.02% C. it is desirable that one or more carbide-forming elements as niobium, tantalum, zirconium, titanium or vanadium be added to give a total content of these elements within the range of 0.3-2%.
Thus, it has remained very desirable to enhance abrasion and erosion resistance of high silicon steels for use in hot concentrated sulfuric acid by means other than the precipitation of hard silicides or the partial transformation of the matrix to hard brittle ferritic phase.